Amplifier



March 22, 1938.. I P HQLST 2,111,589

AMPLIFIER Filed Oct. 31, 1935 Patented Mar. 22, 1938 UNITED STATES PATENT OFFICE V AMPLIFIER Poul F. G.,H lst, Oaklyn, N. J., assignor to Radio Corporation of America, a corporation of Dela ware Application October 31, 1935, Serial No. 47,666

Claims. (01. 250-20) The present invention relates to amplifiers and additional direct current biasing potential from more particularly to electric discharge amplifiers the diode output resistor I l. The rectified signal for, radio receiving systems, phonograph amplicomponent is also applied to the amplifier from fiers and the like, where for some reason the the resistor ll.

5 transmitting device is incapale of reproducing The amplifier device I! is preferably of the over more than a certain limited volume range or high gain high plate impedance type having a dynamic range. For this purpose, the volume or screen grid 3|, a suppressor grid 33, and an outgain through theamplifier is arranged to input anode 35. The output anode is connected crease with increase in the average amplitude through an output circuit 31 and a coupling ca- 0 of applied signals. pacitor 39 with a volume control potentiometer 4| An object of the present invention is to provide to a movable volume control contact 43 of which an improved amplifier of the above type for radio a further amplifier device 45 is connected. The receiving apparatus, phonograph amplifiers and volume control device is preferably of the type the like, wherein the. volume range is expanded having a tone compensating network 41 in conor dynamically amplified without complicated nection therewith. 15 circuits or apparatus. It will be noted that the diode output resistor It is a further object of the present invention II is provided with a by-pass capacitor 59 which to provide a dynamic amplifier or volume exserves to by-pass the intermediate frequency carpander circuit for radio receiving apparatus, rier. Likewise the by-pass capacitor 29 is of such phonograph amplifiers and the like, which is of low impedance value that it serves to by-pass the low cost and involves a minimum of apparatus. audio frequency currents tending to fiow through 20 It is a still further object of the present inventhe bias potential source 21. tion to provide an improved and simplified volume The diode biased amplifier I1 is provided with expander system for radio receiving apparatus, a variable load device in addition to the fixed phonograph amplifiers and the like. loading of the volume control device 4i through 25 The invention will be better understood from an audio frequency transformer 49, the primary the following description when considered in con- 5| of which is utilized as a coupling choke coil in nection with the accompanying drawing and its connection with the output circuit 31 as shown scope will be pointed out in the appendedclaims. and the secondary 53 of which is connectedwith In the drawing, Figure 1 is a schematic cira device 55, the impedance of which increases 30 cuit diagram of a portion of a radio receiving when the wattage dissipation in the device insystem embodying the invention, and creases. The plate circuit of the amplifier tube Figs. 2 and 3 are similar schematic circuit diais, therefore, loaded through a Suitable tc grams of modifications of the circuit of Fig. 1 ing transformer with the device 55 which proillustrating further embodimentsof the invenvides an increasing impedance with increased 35 tion. power consumption. Referring to Fig. 1, 5 is the second detector of a With this arrangement the amplification of ,superheterodyne receiver and is provided by a the combined detector amplifier will vary apdiode rectifier having-an anode I and a cathode 9 proximately proportionally to the load impedance 40 connected through -a diode output resistor ll provided by the device 55 and it will be seen vol- 4 with an intermediate frequency amplifier outume range expansion will be provided. In other .put transformer 13. The diode circuitis groundwords, the increased signal strength provides an ed as indicated at l5. increased audio frequency output, greater watt- The rectified signals are applied to an audio age dissipation, which in turn causes the imfrequency amplifier tube- I! througha tap conpedance of the load 55 to increase, resulting in 45 nection ,l .II on the output resistor II and a lead a magnified gain, and a dynamically amplified 2| connected with the control grid 23 ofthe signal is applied to the volume control device 4|. amplifier l I. The cathodeof the amplifier tube The transformer 49 together with load 55 iniindica'ted at 251s. connected through a source of tially or normally provides a relatively low im- 50 biasing potential such as a. self-biasing resistor pedance for reduced signal gain or volume level. 50 I 21 to ground I5. The resistor 21 is provided with The volume expansion circuit will introduce no a suitable by-pass capacitor 29. distortion if the variable impedance is ar- I The arrangement is such that the control grid anged to have a time s t of s h a magni- 23is provided with'a direct current biasing povtude that the impedance of 55 remains substan- 5ftential fromthe source 2'! as an initial bias and tially constant over the time required to go 55 through one-half cycle of the lowest useful frequency which it is desired to amplify. With the time constant of the impedance device 55 will be understood the time required for 55 to substantially adjust its impedance from the value corresponding to the wattagedissipation to the impedance corresponding to a suddenly applied different wattage dissipation. It will be noted that this time constant will express how fast the expanding device is going to work, and that by a suitable choice of the device 55 may make the operation of the expander fast or slow at will. within the limitations outlined above.

Measurements of a 2 volt, 60 milliampere pilot lamp have shown that the resistance varies as much as 6.4 to 1 for various dissipations of energy in the filament and this has led to the use of such a device at 55 with the filament 51 directly connected across the secondary 53 of the matching transformer. In this circuit the load of the filament 51 changes the amplification in accordance with the wattage dissipation. However, it will be seen that the action is somewhat dependent upon the effective series resistance of the transformer 49 which will be found to be inserted in series with the variable impedance thereby lessening its eifects. Accordingly, a circuit is shown in Fig. 2 wherein the same reference numerals designate the same parts as in Fig. 1.

Referring to Fig. 2 it will be seen that the amplifier output circuit 31 is provided With a coupling transformer 6|, the secondary 63 of which is connected with the volume control potentiometer 4| and is further provided with a low voltage tap between which and the low potential terminal Bl the filament 51 of the lamp 55 is connected.

This arrangement effectively eliminates the primary load impedance of the transformer in the load circuit but it still leaves the secondary load impedance in series therewith. The output circuit of the diode biased amplifier I1 is therefore effectively variable loaded without introducing the impedance of the transformer to any appreciable extent since the portion of the secondary included between the lamp terminals is relatively low. In other words a step down ratio is obtained between the anode circuit of the amplifier and the variable wattage dissipation device while at the same time the secondary is utilized as a coupling means for the succeeding fixed load provided by the volume control device.

Referring now to Fig. 3 in which the same reference numerals are used for similar circuit elements as in Figs. 1 and 2, the diode biased amplifier I1 is provided with a coupling transformer 69 in the output circuit 31, the secondary II of which is connected through a link circuit 13 with a primary winding 15 of a second coupling transformer H, the secondary 19 of which is connected with the volume control device 4| whereby it is loaded.

The secondary H of the input coupling transformer and the primary 15 of the output coupling transformer for the link circuit 13 are both of relatively low impedance adapted to match the impedance of a low resistance variable loading device 55 which may be any suitable device of the type referred to hereinbefore and which is shown again in this embodiment, as a lamp, the filament 51 of which is connected directly across the link circuit 13. One side of the link circuit may be grounded as indicated'at 8|.

With this arrangement, the use of the additional transformer 75-79 makes it possible to utilize the full impedance variation of 55, inasmuch as only the voltage across 55 is transmitted to the succeeding amplifier and the transformer impedance adds only to the plate impedance of the preceding amplifier I1.

When utilizing a circuit as shown in Fig. 3 and a lamp of the 2 volt 60 milliampere type having a cold resistance of approximately 5.5 ohms and a hot resistance of 35 ohms, it was found that the relation between the input voltage to the grid of i1 and the output wattage to the volume control device 4| was as follows:

Input level DB Output level DB from which will be seen that effective volume range expansion or dynamic control of 14 DB is obtained.

It will further be noted that the contact I9 on the diode output resistor may be variable whereby the input signal to the amplifier may be controlled and adjusted to a desired level to initially obtain at the terminals of the volume control device 4| a predetermined signal strength with a given input signal voltage in the diode resistor The system also preferably may include automatic volume control means preceding the diode rectifier. However, as such systems are now substantially universally used in connection with radio receivers it may be assumed that the signals apply to the intermediate frequency output transformer of substantially constant amplitude.

From the foregoing description, it will be seen that in a radio receiver channel or the like, an amplifier wholly or partially diode biased and directly fed from the diode is provided with a variably loaded output circuit. Furthermore, the amplifier is of the high gain high impedance type, and the output circuit thereof is suitably loaded by a variable impedance device, the impedance of which increases with the wattage dissipation. With an increase in applied signal amplitude, the higher impedance across the circuit thereby secures an increase in the gain of the amplifier stage.

The combination of a low impedance variable load in connection with an amplifier tube having a high anode impedance permits the signal voltage transmitted through the load circuit to vary in accordance with the impedance of the load. Accordingly, the loadcircuit of Fig. 3 is particularly desirable for use as a volume expander circuit. In other words, the load circuit gives poor regulation of voltage in response to increasing load because of the series impedance of the plate and transformer which is relatively high with respect to the load impedance.

By utilizing a lamp having a suitable filament the time constant of the expanding function may be controlled, that is, the time of change from one impedance value to another may be adjusted to preventing too rapid change in volume by adjusting the time constant of the load device.

The advantages in the use of a system embodying the invention may be shown by reference to a preferred application to radio receivers and phonograph amplifiers. As is well known,

phonograph records and radio programs are so 7 monitored that the output volume level may not exceed certain limits to cause over-modulation of radio programs or damage to the recording groove of a record. This may result in appreciable limitation of the actual volume range variation of a symphony orchestra being broadcast or recorded.

With the present system, it is possible to obtain the benefit of volume range expansion whereby the dynamic range of the original reproduction is restored to a considerable extent without involving inv the receiving or amplifying apparatus a complicated circuit or costly means, since the cost of radio receiving apparatus must be kept at a low level. This system provides means for including the benefits of Volume expansion in commercial apparatus of that character. In addition .a relatively wide range of expansion control is obtainable.

' I claim as my invention:

1. A volume range expander system including in combination an electric discharge amplifier device of the pentode type having a relatively high anode impedance, an output circuit for said amplifier and means coupled with said output circuit providing a variable low impedance load therein which decreases with wattage dissipation, the impedance of said load being normally relatively low and in shunt relation with the output circuit and said anode impedance providing a series high impedance in said output circuit and efiectively in series with the load, and said impedance being of such high value in series with said load that the signal output voltage r transmitted through the load circuit is permitted to vary in accordance with the impedance of the load, and said load comprising a filamentary resistance device. p

2. In a radio receiving system, the combination with a diode signal rectifier, of a high gain, high anode impedance electric discharge amplifier device and a control grid connected with said diode to receive biasing and rectified signal potentials therefrom, a volume control circuit coupled to said amplifier to receive amplified signals therefrom, said circuit having an impedance of such low value and said high anode impedance being of such high value and being effectively in series therewith as a single controlling impedance that a variable load thereon effectively varies the potential across said circuit, variable load means connected with said circuit comprising a filamentary resistor, the impedance of which increases with increased signal energy applied thereto from said circuit, and means providing a high impedanceoutput circuit for deriving a signal from said amplifier device.

3. In a radio receiving system, the combination of an audio frequency amplifier stage including an electric discharge amplifier device having a control grid, a cathode and an output anode, said amplifier device being of the high gain, high impedance type, means providing a low impedance load circuit transformer coupled with said anode and including an impedance which increases with increased wattage dissipation therein and said amplifier device having an anode impedance sufficiently high to provide a series regulating impedance in said load circuit whereby the signal voltage transmitted through the load circuit may vary substantially proportionally to the load impedance without distortion, and said load impedance having a time constant effective to cause the impedance to be substantially constant over one half cycle of the amplified alternating current signal.

4. In a radio receiving system, the combination of an audio frequency amplifier stage including an electric discharge amplifier device having a control grid, a cathode and an output anode, an output circuit connected with said anode, means providing a load circuit coupled with said output circuit including an impedance which increases with increased wattage dissipation therein, said anode impedance being efiectively in series with the load circuit and being of such relatively high value that the signal voltage transmitted through the load circuit may vary substantially proportionally to the load impedance, said load impedance having a time constant eifective to cause the impedance to' be substantially constant over one half cycle of the amplified alternating current signal, and means for applying signals to said control grid comprising a diode signal rectifier, an output resistor therefor and means providing a direct connection between said control grid and cathode and said output resistor whereby said amplifier is biased in accordance with signal strength variations to vary the eifect of said load impedance.

5. In an audio frequency signal transmission circuit, the combination of an electric discharge amplifier device of the pentode type, means for applying signals thereto, a signal output circuit therefor wherein the anode impedance of said device provides a relatively high series output impedance, a low impedance load circuit coupled to said output circuit whereby the high anode impedance of said device is applied serially therein as a single regulating impedance, means providing a load in shunt with said load circuit, the impedance of which increases with increased wattage dissipation therein, the impedance relation between said anode impedance and the load impedance being such that the high anode impedance in series with the load permits a signal output voltage transmitted through the load circuit to vary in accordance with the impedance of the load.

POUL F. G. HOLST. 

